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  <div class="question_difficulty">
   难度：Easy
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   <h1 class="question_title">
    1. Two Sum
   </h1>
   <p>
    Given an array of integers, return
    <strong>
     indices
    </strong>
    of the two numbers such that they add up to a specific target.
   </p>
   <p>
    You may assume that each input would have
    <strong>
     <em>
      exactly
     </em>
    </strong>
    one solution, and you may not use the
    <em>
     same
    </em>
    element twice.
   </p>
   <p>
    <strong>
     Example:
    </strong>
   </p>
   <pre>
Given nums = [2, 7, 11, 15], target = 9,

Because nums[<strong>0</strong>] + nums[<strong>1</strong>] = 2 + 7 = 9,
return [<strong>0</strong>, <strong>1</strong>].
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   <p>
    &nbsp;
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   <h1 class="question_title">
    1. 两数之和
   </h1>
   <p>
    给定一个整数数组
    <code>
     nums
    </code>
    &nbsp;和一个目标值
    <code>
     target
    </code>
    ，请你在该数组中找出和为目标值的那&nbsp;
    <strong>
     两个
    </strong>
    &nbsp;整数，并返回他们的数组下标。
   </p>
   <p>
    你可以假设每种输入只会对应一个答案。但是，你不能重复利用这个数组中同样的元素。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre>给定 nums = [2, 7, 11, 15], target = 9

因为 nums[<strong>0</strong>] + nums[<strong>1</strong>] = 2 + 7 = 9
所以返回 [<strong>0, 1</strong>]
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